Lots of good advice in this thread. I came from the same place: 133 Lb up to 205 Lb (at the moment). Here's my take.
The first key to it all is what you eat and how much. Don't underestimate that. Most people DO NOT eat the amount of calories they think they are eating when trying to grow. Food volume does not equal caloric value. You MUST get more calories than you burn. Also get lots of protein. Try to eat every 3 hours, alternating between meals and carb/protein shakes. Even if you get a little fat along the way, don't worry, if the long term goal is size. You can worry about dieting later, but with research you will find there are foods & supplements you can add that will help you grow and stave off the fat (fish oil, r-ala, l-carnitine, and poliquin's fenuplex, for examples). My rule: a 6 pack is only good for getting a straight guy drunk.
The other key is consistency in your training. You don't have to train every single day. In fact, you shouldn't. But over the course of time, you need to be regular about getting to the gym. It's a long term process. Personally, I believe heavy weights and lower reps build bigger muscles. Not stupid heavy, but heavy that maxes you out safely, since you need to be concerned about joints, ligaments, and tendons as well. Some body parts are going to just pop out, others are going to lag. Which ones depend on your genetic map. For example, I am unblessed with small calves, so they get extra priority and slowly-slowly, it's paying off. On the other hand, I hardly train my back and tris and they keep growing. So find a strategy to work the lagging parts. Lots of guys neglect their legs. Do your legs. Symmetry is really important. No one wants to look like (or look at) a turnip. Read up and try out different routines to find what works for you. I think it takes 4 to 8 weeks to really see how a routine goes. Change things around periodically so you don't get "stale" for lack of a better word.
Steroids? Optional, but definitely not in the first few years. Get your training figured out first. It'll pay permanent dividends. I wasted the first 10 - 15 years, now I am catching up. Otherwise, you'll be pissing away your roid money. Literally. Meanwhile, consider fadogia agrestis (better) or tribulus terrestris for a natural, relatively safe way to get your body to produce more testosterone. I'd say skip pro-hormones. Pro-hormones are basically 1 step away from real steroids. In other words, they they transmute into steroids in your body and have the same effects and side effects, but because they come from supplements companies and you have to take them frequently (they are not long-acting like some steroid depots), they will rape your budget.